The present invention relates to an electric hand tool, in particular a drill or a screwdriver, comprising a housing, an electric motor accommodated therein and having a motor shaft, a tool shaft which is mounted in the housing and which is driven by the motor shaft via a transmission, and a hand-operated locking mechanism for blocking the rotating movement of the tool shaft.
Such an electric hand tool is known from the British patent application GB 2 304 067 A. In this known electric hand tool the transmission is formed by a drive pinion which is arranged on the motor shaft and which co-acts with a toothed wheel connected non-rotatably to the tool shaft. Provided in the toothed wheel are axial bores in which a locking pin displaceable in axial direction can be placed to block the rotating movement of the tool shaft. The locking element is displaced in axial direction by a rotary knob to be operated from outside the housing. In order to enable exchange of a tool the locking pin is displaced axially relative to the tool shaft by means of the rotary knob until it drops into one of the axial bores of the toothed wheel and thereby blocks the tool shaft.
The object of the present invention is to provide an electric hand tool which is provided with an improved locking mechanism for blocking the rotation movement of the tool shaft. Another objective of the present invention is to provide an electric hand tool provided with a locking mechanism which takes a structurally simple form and necessitates only minimal modification of the existing construction of the hand tool.
To this end the electric hand tool according to the present invention is characterized in that the locking mechanism acts on the motor shaft. The motor shaft lies at a greater distance from the tool shaft than the point at which the locking mechanism of the known hand tool engages. Owing to the greater distance (arm) it is possible to suffice with a lesser locking force in order to block the tool shaft with the same locking effect (moment).
The locking mechanism preferably comprises a first locking element connected non-rotatably to the motor shaft and a second locking element displaceable relative to the housing in the direction of the first locking element. The point of contact of the locking mechanism hereby comes to lie at a distance from the motor shaft and the arm is made even longer. An even smaller locking force is required to obtain the same locking moment. The locking mechanism can therefore be of lighter construction, which results in an economic advantage.
An activating element to be operated from outside the housing is provided for displacement of the second locking element in the direction of the first locking element.
The activating element is preferably received slidably in the housing so that the activating element is guided by the housing.
In order to unblock the rotation movement of the tool shaft after exchanging the tool, the second locking element is biased in a direction away from the first locking element. When the activating element is no longer being operated, the second locking element will return under the influence of the bias to its starting position and disengage from the first locking element.
In an advantageous embodiment according to the invention a resilient member is arranged between the activating element and the second locking element. The activating element to be operated by the user is hereby not in direct contact with the second locking element, so that when the locking mechanism is operated the user is less affected by the reaction force generated by engagement between the first and second locking element. The resilient member partially takes over the reaction force and then transmits it to the housing.
If the hand tool further comprises a hammer mechanism for periodic displacement of the tool shaft in axial direction, which mechanism is actuated via an activating element accessible from outside the housing, the hand tool according to the present invention is further characterized in that the activating element for actuating the hammer mechanism is also used to actuate the locking mechanism. In this way use is advantageously made of existing structural parts already present in the hand tool.
The activating element is then preferably displaceable via a first control member in the direction of the motor shaft and via a second control member in tangential direction of the tool shaft. The second control member has two positions, a first position for drilling and a second position for hammer drilling. In both the first and second position of the second control member the locking mechanism according to the present invention can be activated by operating the first control member.